This invention relates generally to fiber optic networks, and more specifically, to full duplex fiber optic networks that utilize a single multimode fiber optic cable and employ a network architecture that allows for redundancy by creating multiple network backbones operating in parallel or parallel paths on a single network backbone.
Fiber optics provide many technical advantages in terms of weight, volume, power, bandwidth, future proofing (speed and protocol independence), safety (electrically passive, not an ignition source), and is the preferred solution for higher level Electromagnetic Interference and lightning environments in a composite aircraft. However, high component cost and special installation/maintenance burdens of avionic fiber optic components limit its deployment only to where very high-speed communication links are required. Currently each avionic fiber optic data link requires two fiber optic cables, one fiber optic cable for transmitting data and one fiber optic cable for receiving data with both fiber optic cables using the same optical wavelength. In addition, each data link requires many expensive aircraft-grade connectors for production breaks and wiring integration.
There are two main types of fiber optic cables: single-mode and multimode. Single-mode fiber optic cable has a core diameter of 9 μm and only supports a single light propagation mode. Multimode fiber optic cable has a core diameter of 62.5 μm and supports multiple propagation modes. Due to its small core, single-mode fiber optic cable is very susceptible to mechanical misalignment from vibration, shock and thermal excursion. It is also more susceptible to contamination from moisture, dust, a variety of fluids, and fiber endface micro cracks/pits etc. The harsh environmental conditions experienced on aircraft dictate the use of multimode fiber optic cable and optical connectors, which have wider alignment tolerances, in order to be economical and safe.
The term single fiber optic cable should not be confused with single-mode fiber optic cable. A single fiber optic cable bidirectional link means a single strand of fiber optic cable is used to simultaneously support both the transmission and receivership of data in opposing directions. Currently within the telecom industry, this is done almost exclusively with single-mode fiber optic cable.
Bidirectional transceivers are designed to transmit laser light at one wavelength and receiver laser light at a different wavelength than the transmit wavelength. For example, a common wavelength set for bidirectional transceivers could be to have the end user device transmit and receive at wavelengths of 1550 nm and 1310 nm, respectively, and have the switch to which the end user device is connected transmit and receive at wavelengths of 1310 nm and 550 nm, respectively.
Bidirectional transceivers using two different wavelengths for transmitting and receiving data on the same fiber optic cable can eliminate 50% of the fiber optic cable, optical termini, and optical connectors. However, commercial bidirectional transceivers that are affordable due to mass-market volume are designed for the long distance telecom market. These COTS (Commercial Off The Shelf) bidirectional transceivers are configured for use with single-mode fiber optic cable that is typically 2 km to 20 km in length. These bidirectional transceivers come in many varieties including, but not limited to, point-to-point, point-to-multipoint, single frequency laser, and multi-frequency lasers. There are some custom prototype bidirectional transceivers configured for multimode fiber optic cable, but these are not mainstream, and therefore not only expensive and not regulated by industry standards, but also not viable for the long term avionics market.
The other type of laser commonly used in the telecom market is the DFB (Direct Feedback) laser, which not only is more expensive but possesses a single frequency and therefore is not particularly suitable for multimode fiber optic cable applications. Use of a single laser mode (single spectral peak) like that of a DFB laser can excite just a few dominant lower-order propagation modes in a multimode fiber. Theoretically, any two modes can interfere destructively and result in significant power loss. The optical power variation penalty due to the effects of multi laser mode hopping, propagation mode cancellation, and differential mode delay may have significant impact on ultra high speed and very long distance telecom networks, but not on aircraft communication networks. Not only does the short aircraft fiber optic cable have more link budget and higher sensitivity when used with multi-frequency VCSEL (Vertical Cavity Surface Emitting Laser) and FP (Fabry Perot) lasers, but the modal fluctuation dynamic occur at a much faster rate than the overall data pulse width, and therefore the aggregated effect is insignificant.
One skilled in the art may point out another problem with using multimode fiber optic cable is that the Gigabit Ethernet standard requires the use of a special mode-conditioning patchcord for multimode fiber optic cable when transceivers designed for single-mode fiber optic cable are used. The patchcord is a special adaptor that fuses a segment of multimode fiber optic cable and a segment of single-mode fiber optic cable together with the single mode fiber slightly offset from the center of the multimode fiber to excite more propagation modes in the multimode fiber. The patchcord requires the use of two separate fiber optic cables for transmitting and receiving data since the patchcord only works in the transmit direction. The patchcord does not work in the receiving direction since a great deal of optical power is lost in transitioning from the larger multimode fiber optic cable to the smaller single-mode fiber optic cable. Mode conditioning patchcords are therefore used with separate transmit and receive fiber optic cables and are not suitable for single fiber optic cable applications.
Others skilled in the art would likely point out that variations in connector losses are caused by under-filled multimode fiber optic cables where the core modes may migrate off to one side or quadrant (cross section) of the fiber optic cable. If this quadrant is slightly offset in the butt-joint connector, then a significant amount of the optical signal may be lost. This can be a major problem with single mode fiber optic cable where the cable diameter is only 9 μm. However, the mode selective loss due to connector offset is insignificant for multimode fiber optic cable. Published measurement of over 3,000 multimode fiber optic cable connectors from different suppliers indicate a mean offset of 3 μm, which is small compared to the 62.5 μm core diameter.